BJ11 says... Hello,
Never fear, mulberries are weeds - it will recover well after frost. To minimise impact water with some sea-sol, encourage good fungi in your soil (add home-made compost a few times per year), and if you're really worried give it a little shelter and water the night before a big frost is due.
Mulberries come from regions that get colder than anywhere in Oz. They are tough and recover if they are singed!